Threads on plastic parts are generally divided into internal threads and external threads. For external threads, two sliders are generally used to form or half are formed using a fixed mold plate and a movable mold plate. For internal threads, there are three different methods for demoulding: After molding, there are forced demoulding, manual demoulding and automatic demoulding. Forced demoulding is only suitable for plastic parts with soft materials; manual demoulding is to manually rotate thread core or plastic parts to demould internal threads. This demoulding structure is simple, but efficiency is low and labor intensity is high, so it is not suitable for mass production. Automatic demoulding is to install a hydraulic motor (or hydraulic cylinder), gear mechanism, worm gear, etc. on mold to form an internal thread demoulding mechanism, so that threaded core automatically rotates and molded plastic parts are automatically demoulded. Structure of automatic demoulding mechanism is complex, but due to its stable and reliable operation, it can automatically demould internal threads on plastic parts and is widely used. Now, taking a pipe joint with internal and external threads as an example, we will introduce structure of mold for forming it.

Pipe joint plastic part is cylindrical, with a height of 149mm, a maximum outer diameter of φ69mm, and a wall thickness of 3mm, as shown in Figure 1. There are threads on both ends of plastic part, one end is an external thread with a specification of M64mm and a length of 18mm; the other end is an internal thread with a specification of M24mm and a length of 16mm. Plastic parts are made of ABS, which is a hard material, neither internal nor external threads can be forcibly demoulded. Four reinforcing ribs are evenly distributed on outer surfaces of both ends of plastic part. In addition to strengthening strength of plastic part, reinforcing ribs also prevent plastic part from rotating along with internal thread core when internal thread is demoulded.

 

According to structure of threaded pipe joint, a two-plate mold pouring system is used. Gate is set at the edge of R-curved surface of outer surface of plastic part to be formed, and a latent gate is used for pouring, as shown in Figure 2(a). If main channel is too long, its diameter will increase accordingly, and it will take a long time to cool molten material in main channel. Therefore, parting surface of mold is suitable to be set at arc of plastic part. Main channel is located in sprue sleeve, and branch channel is located on upper surface of movable platen. A cold slug cavity is set directly below main channel, and side wall of cold material cavity is set with a slope of 10°, and use this slope to pull gate condensate out of main flow channel. When two sliders of molded plastic part are separated, runner condensate is connected to plastic part. In order to eliminate weld marks on the surface of plastic part, two ends of runner are slightly extended, as shown in Figure 2(b).

 

According to structure of plastic part and requirements for mass production, mold adopts a 2-cavity structure. Internal thread of plastic part is formed on internal thread core located in fixed mold. In order to automatically demould internal thread, a transmission mechanism of "hydraulic cylinder + rack and pinion + gear transmission + threaded nut" is installed on fixed mold side. External thread and external cylindrical surface of plastic part are formed by collision of two slide blocks, and slide grooves of the two slide blocks are opened on pusher plate. Since plastic part is a pipe joint and is hollow inside, it is suitable to use a push plate to push it out from core during demolding instead of a push rod. Mold structure is shown in Figure 3. Mold working process is as follows.

 

1. Moving mold seat plate 2. Push plate 3. Push rod fixed plate 4. Pad block 5. Backing plate 6. Core fixed plate 7. Push rod 8. Push plate 9. Push plate insert 10. Moving mold plate 11. Guide sleeve 12. Guide column 13. Fixed mold insert 14. Rolling bearing 15. Driving gear 16. Rack 17. Fixed mold base plate 18. Pinion 19. Drive shaft 20. Nut 21. Threaded core 22. Water pipe Joint 23. Fixed mold plate 24. Slider 25. Core 26. Slider spring 27. Inclined guide column 28. Seal ring 29. Limit column 30. Return spring 31. Return rod 32. Hydraulic cylinder 33. Gate sleeve 34 .Positioning circle diagram
Figure 3 Mold structure
(1) After injection is completed, before fixed mold and movable mold are opened, piston rod of hydraulic cylinder 32 contracts and drives rack 16 to move. Rack 16 drives driving gear 15 to rotate, driving gear drives two threaded cores 21 to rotate. Under action of threaded nut mechanism, threaded cores 21 rotate while rising to demold internal threads of plastic part.
(2) After internal thread of plastic part is demoulded, moving and fixed molds are separated. At the same time, inclined guide pillar 27 drives two sliders 24 to demould. At this time, plastic part and flow channel condensate remain on core 25.
(3) Slider of injection molding machine drives mold ejection mechanism (push plate 2, push rod fixed plate 3, push rod 7, push plate 8, push plate insert 9, etc.) to push plastic part and flow channel aggregate out of core 25.
(4) Slider of injection molding machine cancels force exerted on mold ejection mechanism, and return spring 30 drives mold ejection mechanism to reset.
(5) Under action of slider of injection molding machine, movable and fixed molds begin to close mold. During mold closing process, two sliders 24 are reset at the same time.
(6) After mold is completely reset, next plastic part can be injected.

External threads and external cylindrical surfaces on plastic parts are demoulded using inclined guide posts + springs + sliders, as shown in Figure 4. Due to small size of slider, and in order to enhance strength of slider, an overall structure is adopted. Arc surface of plastic part and outer cylindrical surface corresponding to internal thread are formed by fixed mold, so height of slider is lower than height of plastic part in mold. In order to make movement of slider smooth, two springs are set on slider. When mold is opened, two springs push slider away. In order to extend service life of slider, wear-resistant sheets should be installed on slope of slider. Slider material is 718, and cavity wall on slider needs to be nitrided.

 

Focus of mold structure is to automatically demould internal threads. Method is to set gear and thread structure in threaded nut mechanism on internal thread core, as shown in Figure 5(a), install hydraulic cylinder, rack 16, pinion 18, driving gear 15 and nut 20 on mold etc. One driving gear 15 drives two threaded cores 21 to rotate at the same time, as shown in Figure 5(b). Under action of nut 20, thread core rises along axis direction while rotating, so that internal thread of plastic part is demoulded.

 

15. Driving gear 16. Rack 18. Pinion 19. Transmission shaft 20. Nut 21. Thread core diagram
Figure 5 Automatic demoulding mechanism

According to structure of plastic part, a pusher plate is used to push plastic part out, as shown in Figure 6. Push-out system includes push plate 2, push rod fixed plate 3, push rod 7, push plate 8, push plate insert 9, limit column 29, return spring 30, return lever 31, etc. When movable mold and fixed mold are separated, two slide blocks 24 are separated synchronously, then slider of injection molding machine drives push plate 2 and push rod fixed plate 3 of mold to move, so that push rod 7 drives push plate 8 and push plate insert 9 to move, push plate insert 9 pushes molded plastic part from core. It should be noted that pusher plate 8 and pusher plate insert 9 must move together, because there is no special guide post guide bushing on pusher plate insert 9, otherwise pusher plate insert 9 will easily get stuck during movement.

 

2. Push plate 3. Push rod fixed plate 5. Backing plate 6. Core fixed plate 7. Push rod 8. Push plate 9. Push plate insert 24. Slider 29. Limiting column 30. Return spring 31 .reset lever diagram
Figure 6 Launch system

Fixed mold adopts a straight-through cooling water channel and uses a quadrilateral water channel to surround two mold cavities. Since fixed mold cavity has an insert structure, water pipe joint is installed on the side of fixed mold plate, as shown in Figure 7(a); since two sliders are molded parts of external thread and external cylindrical surface of plastic part, and are large in size, in order to maintain uniform mold temperature, a cooling water path is designed on slide block. According to structure of plastic part, slider is suitable for using a waterway that combines "straight-through and water wells". Each slider is equipped with three water wells, and three water wells are connected in series with a straight-through waterway, as shown in Figure 7(b). Two dynamic model cores are cylindrical and are suitable for use of spacer-type water wells. Diameter of water well is φ16mm. Water pipe joints are connected through pad at the bottom of movable mold core.

 

In order to demould internal threads of molded plastic parts, hydraulic cylinder, rack and pinion mechanism, gear transmission mechanism, threaded nut mechanism, etc. are installed on mold to form an automatic internal thread demoulding mechanism. Under action of piston rod of hydraulic cylinder, thread core rotates and rises, so that internal thread is demoulded. For this pipe joint, a two-plate mold structure with 2 cavities is adopted. Since mass production, mold movement has been stable and reliable, and molded plastic parts have a high pass rate.